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POstd::string in struct - Copy/assignment issues?
 

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  1. POstd::string in struct - Copy/assignment issues?
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    <p>Suppose I have a struct containing a std::string, like this:</p> <pre><code>struct userdata{ int uid; std::string username; } </code></pre> <p>Do I need to create a copy ctor or anything to return it from a function or to use it inside a STL container? Consider this function:</p> <pre><code>userdata SomeClass::GetUserData(unsigned int uid) { //do error checking and other stuff... //m_usermap is std::map&lt;unsigned int, userdata&gt; return m_usermap[uid]; } </code></pre> <p>When I insert userdata structs into the std::map, a copy of the struct gets created, right? Does a new std::string get created using the value of the username field, or does some sort of bitwise copy happen (this would be bad)? Similarly, when I return a userdata struct from the GetUserData method, does it have an independent string holding the username or do I need to define a copy ctor and explicitly create a new string?</p>
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    1. CORemember that 'struct' and 'class' *interchangeably* define class-types, with the only difference being public or private default access. (It's a common misconception, even though you didn't directly say you believed this.) This means userdata has a copy ctor even though you didn't explicitly declare one (*all* class-types have a copy ctor), and the compiler-generated one will do a member-wise copy, which calls the copy ctor of each member, and that's how alemjerus's answer below applies. All compiler-generated copy ctors and assignment operators behave member-wisely instead of bit-dumbly.
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    2. COWhen in doubt, write copy constructor and assignment operators for the `struct` or `class`. I prefer not to rely on *hidden* activities of the compiler.
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      1. USThomas Matthews
    3. CO@Thomas: I completely disagree with that advice. The compiler generates ones do a perfectly good job. Unless you know what they are doing a novice is going to make a mistake, while somebody who does know what they are doing does not need to do anything. So unless the class contains a pointer the default methods are absolutely fine and probably better than a novice could write. Additionally less code means less maintenance costs.
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      1. USMartin York
 

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